Had the same set up when I started, first 2 plants were 2 oz a piece. The next few I got 3-4 oz with mid sized strains . So it’s definitely not the lights , cobs work well but I’ve moved on to strips & qbs . Are you pushing out the hot air & sucking in fresh air? Temps could be 5 degrees cooler , mine stay @ 84-88f with lights on & do well. So something else might be the problem unless you’re using SOG type plants that grow small . What’s your full set up ? With more information I’m sure someone could help you out .